Nowadays paper is as a rule coloured with cationic direct dyes, which are water insoluble dyestuffs with strong affinity to cellulose fibres. The dyestuff is generally added to the fibre dispersion, i.e. the fiber dispersion in water which is applied onto the wire, but it can also be applied onto the completely formed paper sheet by means of various application methods. These colouring methods have in common that they give a dyeing of all pulp fibres containing reactive groups whereby a comparatively evenly coloured paper sheet is obtained. In manufacture of paper based on recycled fibres it occurs that coloured paper is reused and thereby a recycled paper pulp is obtained with an element of coloured fibres giving a paper with a mottled colour.
Coloured nonwoven materials can be manufactured in different ways, e.g. by addition of a certain proportion of coloured synthetic fibres. This is however comparatively expensive because of the high price involved with coloured fibres. A cheaper way of colouring nonwoven material is to apply dyestuff by means of some form of finishing treatment such as Foulard treatment or by printing of dyestuff possibly together with a binder. Both Foulard treatment and printing make great demands on accurate dyestuff dosing systems if a result pleasing to the eye is to be obtained.
In certain types of soft tissue paper, e.g. napkins and nonwoven material, a mottled colouring may be preferable because of aesthetic reasons, since a mottled colouring is associated with textile rather than paper.
It is known from U.S. Pat. No. 4,202,852 to manufacture a coloured drylaid nonwoven material by applying dyestuff onto reel or sheet pulp, which is thereafter dried to a moisture content of less than 15% and defibrated in a hammer mill. The fibres are airlaid onto a wire, whereupon a binder is added to the airlaid fibre web. After curing of the binder an evenly coloured drylaid fibre material is obtained.